Vasoconstriction is a narrowing of the blood vessels resulting from contracting of the muscular wall of the vessels. When blood vessels constrict, the flow of blood is restricted or slowed. It is the opposite of vasodilation, the widening of blood vessels. Factors causing vasoconstriction are called vasoconstrictor, also vasopressors or simply pressors. Vasoconstriction usually results in an increase of the blood pressure. Vasoconstriction may be slight or severe. Many vasoconstrictors also cause pupil dilation. Vasoconstriction in the penis can disable males from maintaining an erection (erectile dysfunction). It may result from disease, medication, or psychological conditions. Medications that cause vasoconstriction include antihistamines, decongestants, cough and cold combinations, pseudoephedrine, and caffeine. Heart rate will be 20BPM or more

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Vasoconstriction is mostly the result of increased intracellular concentration of calcium (Ca2+ions). However, specific mechanisms for generating an increased intracellular concentration of calcium depends on the vasoconstrictor. In any case, this calcium results in contraction of smooth muscle (see Smooth muscle for details), resulting in a constriction of the vessel.

Vasoconstriction is a procedure of the body that avoids orthostatic hypotension. It is a part of a body negative feed back loop in which the body tries to restore homeostasis.

For example, vasoconstriction is a hypothermic preventative in which the blood vessels constrict and blood must move at a higher pressure to actively avoid a hypoxic reaction. ATP is used as a form of energy to increase this pressure to heat the body. Once homeostasis is restored the blood pressure and ATP production regulates.

Vasoconstriction also occurs in superficial blood vessels of warm-blooded animals when their ambient environment is cold; this process diverts the flow of heated blood to the center of the animal, preventing the loss of heat.

An increase of blood flow in the penis that builds up causes an erection, however if the body is not functioning properly the blood vessels in the penis activate and allow less blood to pool, meaning the erection goes down prematurely or simply fails to completely go up. This does not necessarily mean that the person in question is not aroused, it is a lapse in judgement between the Central Nervous System [CNS] and the Peripheral Nervous System [PNS] . When blood pools in the penis the body reacts to the excessive heat to cool it, resulting in vasoconstriction.